Disturbance and Community Structure in a Shallow Water Sand Flat

Diopatra cuprea, an onuphid polychaete, is a large and conspicuous component of semiprotected sand flats along the east coast of North America. It builds a large tube which projects above the surface of the sediment. Infaunal abundances are greater around such tubes than elsewhere. To test the hypothesis that tubes provide a local refuge from predation, I manipulated the abundances of the conspicuous predators and sediment disruptors, Callinectes sapidus the blue crab and Limulus polyphemus the horseshoe crab. In the absence of all predators >0.625 cm, infaunal abundances no longer increased as a function of Diopatra abundance. Thus, the tube of Diopatra appears to act as a refuge from predation for infauna. Further manipulations of the predators indicated that Callinectes was far more important than Limulus. Species which were numerically dominant in the presence of predators predominated in the predator removals as well. These experiments were repeated three times over a 3—yr period (1974, 1975, 1976) and in general the results were consistent over that period. Densities of juvenile Diopatra increased dramatically inside predator exclosures. Densities of adult Diopatra did not. The abundance of this refuge—forming organism, therefore, is at least partially dependent upon the frequency with which individuals pass through the size—dependent period of susceptibility.